lol lots and lots and lots of live rock. like hundreds of dollars worht. i know most people will put around 3/4-1 lbs per gallon or sometimes even more. build it up over time find the best stuff you can usually u can find people cleaning out their tank giving it away suppper cheap on craigslist like as low as $1 per lbs but that is rare typically i see it around $3-$4 per lbs but sometimes they will sell a whole lot for super cheap.

2 pounds per gallon is the generally agreed upon optimal amount. Luckily, 3/4ths of that can be cheap base rock, with only the top layer being the good stuff. Indeed, stacked that high, the good stuff would only go to waste if not on top.

That should be enough to fill 1/3rd of your tank with rock ( in visual volume ) which is pretty much perfect. 250 lbs of rock sounds like a lot, but it isn't so much when you are looking at it in your 125 gallon tank.

By the way, you will naturally be sorely tempted to severely cut this corner due to the sheer expense, but I'm telling you now--> Huge mistake. If you want a reef tank that WORKS and doesn't come crashing down or turn into a foul algae pit, you'll have to set it up correctly from the start. I know it hurts, but the alternative is nothing but hassles and headaches for over a year or two.

Well, rock density makes a difference, I suppose. Down here we use heavy Gulf Of Mexico rock, but if you used Fiji rock you'd get the same volume with less weight. ( it just wouldn't be as good looking as Gulf rock. )

EDIT: you could also try making your own rock out of stuff called "Aragocrete." It would be utterly lifeless, but it would be cheap. Go to the GARF site ( www.garf.org ) for details about how to do all sorts of nifty reef stuff. The site is ridiculously hard to navigate, but chock full of goodness. Be ready to make a day of surfing it.

With premium rock I would always go 1 pound per gallon, with the heavier base rock I would go 2-3 pounds per gallon. Also try to get rock that has some shape to it, and remember that most rock that you buy hasn't been cured, or cured correctly. Uncured rock will send a serious spike through your tank. It's best to get the majority of your rock in place, let it go through the spikes, and then add fish. I typically added my live rock at the beginning of the set up and used that for the cycle.

With the advances in skimming and filtering I believe you don't need a huge amount of liverock to be extremely successful. Minimalistic aquascapes are the new rage! I did it in my own tank, and am very pleased with the results. 92 gallon corner about 60lbs of liverock! Go with what YOU personally like as long as you are willing to put in the money on the filtration.